A fuel cell is an apparatus which generates electrochemical reaction in a membrane electrode assembly (hereinafter, referred to as “MEA”.) comprising an electrolyte layer (hereinafter, referred to as “electrolyte membrane”.) and electrodes (an anode catalytic layer and an cathode catalytic layer) arranged on both sides of the electrolyte membrane, and which extracts electrical energy generated by the electrochemical reaction to outside. Among various fuel cells, solid polymer electrolyte fuel cell (hereinafter, referred to as “PEFC”.) used for domestic cogeneration system, automobiles, and so on can be actuated in a low temperature region. Because of its energy conversion efficiency, short start-up time, and small-sized and lightweight system, the PEFC has received attention as a power source of a battery car or a portable power supply.
A unit cell of the PEFC comprises a MEA and a pair of current collectors sandwiching the MEA therebetween; the MEA contains a proton conductive polymer which expresses proton conductance under moisture state. During the operation of PEFC, a hydrogen-based gas (hereinafter, referred to as “hydrogen”.) is supplied to the anode, meanwhile an oxygen-based gas (hereinafter, referred to as “air”.) is supplied to the cathode. The hydrogen which supplied to the anode is separated into proton and electron under the action of catalyst contained in the anode's catalytic layer (hereinafter, referred to as “anode catalytic layer”.) ; the proton generated from the hydrogen reaches a cathode's catalytic layer (hereinafter, referred to as “cathode catalytic layer”.) through an anode catalytic layer and an electrolyte membrane. On the other hand, the electron reaches a cathode catalytic layer through external circuit; by having such a process, it is capable of extracting the electrical energy. Then, reaction of the proton and the electron reached the cathode catalytic layer with oxygen contained in the air which is supplied to the cathode catalytic layer produces water.
During the operation of PEFC, water distribution in the unit cell sometimes becomes uneven. When water distribution in the unit cell becomes uneven, in a dried portion where water is little, proton conductive-resistance of the electrolyte membrane increases whereby the electric power generation efficiency tends to decrease; in a wet portion where water is pooled, the excessive water prevents the diffusion of gas, which tends to lower the electric power generation efficiency. Due to these reasons, in order to improve electric power generation efficiency, homogenization of water distribution in the unit cell is strongly demanded.
As an art for the purpose of homogenization of water distribution in the unit cell, for example, Patent document 1 discloses a fuel cell which comprises a gas separator having a compact layer and a porous portion, wherein the porous portion of the gas separator except for the porous portion is used for preservation of electrolytic solution. In addition, Patent document 2 discloses a fuel cell system comprising a plate where a reactant flow path and a liquid flow path are separated by a porous material, wherein pressure difference is given between upstream and downstream of the reactant flow path, reactant is humidified at the upstream of the reactant flow path and liquid water of the reactant is removed at the downstream of reactant flow path.    Patent Document 1: Japanese Patent Application Examined No. 8-1803    Patent document 2: International Publication No. WO 2004/51818